Synthesis and Color-tunable Luminescence of Ce3+, Tb3+ Codoped Sr6YSc(BO3)6 Phosphor
© Chen and Xia; licensee Springer. 2014
Received: 24 January 2014
Accepted: 25 February 2014
Published: 23 April 2014
Multi-color emitting phosphors will be potential in the fabrication of solid state lighting devices. Color-tunable Sr6YSc(BO3)6:Ce3+,Tb3+ phosphors were synthesized by a high temperature solid-state reaction, and the crystal structure and luminescence properties were investigated in detail. The photoluminescence excitation spectrum of Sr6YSc(BO3)6:Ce3+ show that the excitation peaks from 200 to 400 nm are attributed to the characteristic 4f-5d transitions of Ce3+, and the broad-band blue emission can be also found from the as-synthesized Sr6YSc(BO3)6:Ce3+. Under the excitation of near ultraviolet (n-UV) light, Sr6YSc(BO3)6:Ce3+,Tb3+ phosphors exhibit not only a broad blue emission band originating from the f–d transition of Ce3+ ion but also a group of sharp characteristic green emission lines from the f–f transition of Tb3+ ion, respectively. The excitation spectra monitored at 544 nm emission of Tb3+ consists of the characteristic excitation bands originating from Ce3+ and Tb3+ ions, which proves the occurrence of the energy transfer between Ce3+ and Tb3+. The energy transfer behaviors in Sr6YSc(BO3)6:Ce3+,Tb3+ phosphors is also investigated by the lifetime measurement. The above results indicate that Sr6YSc(BO3)6:Ce,Tb can act as a potential candidate for near-UV-pumped light emitting diodes.
In recent years, along with the energy shortage and global warming, the development of energy saving products in the lighting field, such as white light-emitting diode (WLED), has attracted great attention . Due to the potential applications in indicators, green architectural lighting, automobile headlights, backlights, and general illuminations, WLEDs have become a kind of daily lighting source with excellent properties such as high efficiency, good light stability, long operational lifetime, and environmentally friendly characteristics [2–5]. Furthermore, many efforts have been made in the search of down-converting phosphors converting ultraviolet (UV) or blue light into a combination of red-green-blue light in order to obtain white light emission . Commercial WLEDs lamp is commonly fabricated by using a blue InGaN LED chip and the yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce) phosphor. However, such WLEDs have a poor color rendering index (CRI) and a high correlated color temperature (CCT) [7, 8] because of lacking a red component. However, WLEDs can be also fabricated by pumping blue, green and red phosphors coated on a near-UV (n-UV) LED chip which has a high CRI . For this reason, it is necessary to develop novel multi-color emission phosphors in the field of optical materials.
Recently, borates compounds attracted much attention due to their excellent optical properties, low synthesis temperature, less expensive raw materials, and high physical chemistry stability [10, 11]. Among them, the new compounds with the general formula of A6MM(BO3)6, (A = Sr; M = Gd, Y; M = Al, Ga, In, Sc and Y) has been reported previously [12, 13]. In this work, Sr6YSc(BO3)6 was selected as the host material, Ce3+, Tb3+ singly doped and co-doped Sr6YSc(BO3)6 phosphors were firstly reported. It is well known that Ce3+ ions acts as a good sensitizer in co-doped materials, and transfers a part of energy to the activator ions , such as Eu2+, Tb3+ and Mn2+[14–16]. As for the Ce3+-Tb3+ couples with fixed Ce3+ content, the Tb3+ emission from 5D3 level will be quenched and 5D4 emission will increase gradually with increasing Tb3+ concentration . Herein, the novel color-tunable Sr6YSc(BO3)6:Ce3+,Tb3+ phosphors have been obtained via the traditional solid-state method. Blue to blue-green emission can be realized in this series of phosphors by changing the Tb3+ concentration. Luminescent properties and energy transfer process between the sensitizer (Ce3+) to activator (Tb3+) in the Sr6YSc(BO3)6 host have been studied, and the energy transfer mechanism is also investigated.
A series of Sr6YSc(BO3)6:Ce3+,Tb3+ phosphors were prepared by using the conventional high temperature solid state reactions. SrCO3 (99.5%), Y2O3 (99.99%), Sc2O3 (99.99%), H3BO3 (99.5%), CeO2 (99.99%) and Tb4O7 (99.99%) were used as the starting materials. Stoichiometric amounts of the starting materials were thoroughly mixed and ground thoroughly in an agate mortar. Then, the mixture was transferred into an alumina crucible and calcined in a muffle furnace at 1100°C for 6 h under CO reducing atmosphere. The as-prepared samples were reground into powder for measurement at room temperature.
The crystal structures of the samples were checked by X-ray powder diffraction (SHIMADZU, XRD-6000, 40 kV and 30 mA, Cu Kα λ = 0.15406 nm). Photoluminescence excitation (PLE) and emission (PL) spectra were performed by using a JOBIN YVON FluoroMax-3 fluorescence spectrophotometer with a photomultiplier tube operating at 400 V, and a 150 W Xe lamp used as the excitation lamp. The decay time were carried on using a spectro-fluorometer (HORIBA, JOBIN YVON FL3-21), and the 250 nm pulse laser radiation (nano-LED) was used as the excitation source. And the CIE coordinates were calculated based on the photoluminescence spectra between 375 and 650 nm.
Results and discussions
CIE chromaticity coordinates of Sr 6 YSc(BO 3 ) 6 :0.005Ce 3+ , x Tb 3+ ( x = 0.00, 0.01, 0.03, 0.05, 0.10, 0.20, 0.30) phosphors under the 365 nm excitation
Sample composition ( x) of Sr6YSc(BO3)6:0.005Ce3+, x Tb3+
CIE coordinates ( x, y)
In summary, Sr6YSc(BO3)6:Ce3+,Tb3+ phosphors was synthesized successfully by the conventional high temperature solid-state reactions. The luminescence spectra and decay curves demonstrated that Ce3+ ion can absorb UV photons via the allowed 4f-5d absorption and greatly enhance the green emission of Tb3+ ion under the 365 nm excitation. Sr6YSc(BO3)6:Ce3+,Tb3+ shows obvious absorption peaks including the absorption lines of Ce3+ ions and Tb3+ ions. Due to existence of the energy transfer, the emission hue of the phosphor can be varied from blue (0.1682,0.0934) eventually to bluish green (0.2243,0.3261) with increasing Tb3+ concentration and a fixed Ce3+ content in Sr6YSc(BO3)6. Thus, the obtained Sr6YSc(BO3)6:Ce3+,Tb3+ phosphor is expected to be developed as a suitable phosphor candidate for the application in near-UV excited white LEDs.
This present work was supported by the National Natural Science Foundations of China (Grant No. No.51002146, No.51272242), Natural Science Foundations of Beijing (2132050), the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-12-0950), the Fundamental Research Funds for the Central Universities (2011YYL131), Beijing Nova Program (Z131103000413047) and Beijing Youth Excellent Talent Program (YETP0635).
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